Multilayered absorbent structures

ABSTRACT

A multilayered absorbent structure is disclosed. The absorbent structure has a plurality of absorbent planar regions defined by decreasing pore size with increasing depth into the region. Generally, each planar region has an absorbent layer having relatively large average pore sizes at the top, body facing surface, and relatively small average pore sizes at the bottom, garment facing surface. A subsequent absorbent region has a top body facing surface with an average pore size which is larger than the bottom, garment facing surface of the previous absorbent region. The top surface of each subsequent planar region is in fluid communication with the lower surface of the planar region above.

FIELD OF THE INVENTION

The present invention relates to absorbent structures having multiplelayers. These structures may be used in diapers, adult incontinencearticles, feminine protection such as sanitary napkins, etc. The presentinvention is particularly useful in absorbent products which receive"gushes" of fluids, but it is also useful in products which accept amore continuous application of fluids at a lower rate.

BACKGROUND OF THE INVENTION

Early construction of disposable hygiene products consisted simply of atopsheet, a pulp or tissue wadding core, and an impermeable backsheet.Significant developments in more recent years have included the move toincorporating superabsorbent materials in the absorbent core. However,devices having only a topsheet, a pulp and superabsorbent core, and abacksheet are outperformed by structures which include a transfer layer(fluid management layer) between the topsheet and the core. The transferlayer might be a fabric layer, a pulp/fiber composite, or even a foamlayer. The transfer layer functions to provide a surge capacity forlarge voids to prevent overflow leakage, to provide capillary suction todraw liquid from the topsheet into the absorbent core, and to retard orinhibit liquid from coming back up through the topsheet and onto thewearer's skin.

Therefore, the challenge is to design each individual transfer layer andto optimize the placement sequence of multiple transfer layers in theproduct. The manufacturer of a nonwoven material has limited opportunityto affect the absorbency of the material. However, the manufacturer cancontrol the average pore size of the nonwoven material. The pore size isa characteristic of the material that helps to determine its ability towick fluid and to rapidly transfer fluid, i.e., the permeability of thematerial.

Measurable characteristics of absorbent structures include rewet andstrike-through. Rewet is also described as "surface wetness". It is theamount of absorbed liquid that is detectable at the surface of anabsorbent structure after absorption into the absorbent body. Inmeasuring this characteristic, a structure absorbs a given amount ofliquid, a given pressure is applied to the structure, and the amount ofliquid detectable at the structure's liquid application surface ismeasured. An absorbent structure exhibiting "good" rewet characteristicswould maintain a relatively dry surface, while a structure exhibiting"poor" rewet characteristics would transfer significant liquid backthrough to the surface of the structure.

Strike-through is the time measured for a given amount of liquid to passthrough the facing of an absorbent structure and into its core. Anabsorbent structure exhibiting "good" strike-through characteristicswould quickly accept and absorb applied liquids, while a structureexhibiting "poor" strike-through characteristics would allow appliedliquids to form puddles on its surface.

Practitioners in absorbent structure technology have recognized that apositive density gradient, i.e., decreasing pore size with increasingdepth into an absorbent structure, improves the performance of thestructure. This allows liquids to be accepted into the structure in aregion having large pore sizes. However, to improve rewetcharacteristics, decreasing pore size lower in the structure draws thefluid further into the structure, away from the wearer's skin. Thisprovides a drier surface. It is recognized that decreasing the pore sizeof a hydrophilic material increases its capillary suction for aqueousliquids. This concept is explored in Meyer, U.S. Pat. No. 4,798,603;Cadieux, EP-A-0 359 501; and Kellenberger, U.S. Pat. No. 4,688,823.

Meyer teaches that the pore size should decrease in progressing from thetopsheet to the transfer layer and to the core. In other words, thereshould be a negative pore size gradient or a positive density gradient.Such a configuration provides a capillary-pressure gradient between eachlayer, sucking fluid deeper into the absorbent structure, whilepreventing or reducing rewet.

Cadieux discloses a multiple layered absorbent structure whichincorporates a positive density gradient from its cover sheet, through atransfer layer, and to and including a reservoir layer. The cover sheetis disclosed as a relatively low density, bulky, high loft nonwoven webmaterial. The transfer layer may be composed of fibrous materials, suchas wood pulp, polyester, rayon, flexible foam, etc., and the reservoirlayer is a highly dense absorbent layer having a fine porosity such ascompressed peat moss board.

Kellenberger attempts to achieve a concentration gradient insuperabsorbent particles distributed within an absorbent fibrous mass orabsorbent core. The superabsorbent may be distributed within theabsorbent core in a number of concentration gradients: a positiveconcentration gradient, similar to Cadieux; a bi-nodal concentrationgradient, having maxima proximate the top and bottom surfaces, and aminimum concentration at the center of the layer; and a distributionhaving minimal amounts of superabsorbent proximate the top and bottomsurfaces, and a maximum concentration at the center of the layer. Thisabsorbent core can be enclosed between two creped wadding or tissuesheets and used in an absorbent article further having a top cover sheetand a bottom barrier layer.

Other references have provided for multiple layers of absorbentmaterials in an absorbent structure. These references include Mesek,U.S. Pat. Nos. 4,670,011 and 4,960,477; Iskra, U.S. Pat. No. 4,605,402;Chen, U.S. Pat. No. 5,037,409; Ness, U.S. Pat. Nos. 4,842,594 and4,880,419; Dawn, U.S. Pat. Nos. 4,338,371 and 4,411,660; and Allison,U.S. Pat. No. 4,531,945.

The prior art generally represents the advance of the absorbentstructure art. However, continued advances in this art are needed. Inparticular, a new absorbent structure is needed which will quicklyabsorb body fluids, especially gushes of fluids, and strongly containthe absorbed fluids. Such a structure, if easily and economicallymanufactured, would be very useful in the manufacture of low costdisposable body fluid absorbent articles.

SUMMARY OF THE INVENTION

The present invention relates to a multilayered absorbent structurewhich will quickly absorb body fluids, especially gushes of fluids, andstrongly contain the absorbed fluids. The absorbent structure has aplurality of absorbent planar regions defined by decreasing pore sizewith increasing depth into the region. Generally, each planar region hasan absorbent layer having relatively large average pore sizes at thetop, body facing surface, and relatively small average pore sizes at thebottom, garment facing surface. A subsequent absorbent region has a topbody facing surface with an average pore size which is larger than thatof the bottom, garment facing surface of the previous absorbent region.The top surface of each subsequent planar region is in fluidcommunication with the lower surface of the planar region above.

In one embodiment of the present invention, the absorbent structure hastwo planar regions. Each region has two fibrous layers comprisinghydrophilic fibers. Each of the layers has a respective average poresize. In the top planar region, the average pore size of the lowerfibrous layer is less than the average pore size of the upper fibrouslayer. In the lower planar region, the lower fibrous layer has anaverage pore size which is less than the average pore size of the upperfibrous layer. In addition, the average pore size of the lower fibrouslayer in the top planar region is less than the average pore size of theupper fibrous layer in the lower planar region. In essence, descendingthrough the absorbent structure from the liquid accepting upper surfaceof the top planar region, the average pore size progresses fromrelatively larger to relatively smaller to relatively larger torelatively smaller.

In a particularly preferred embodiment, the absorbent structure iscorrugated by repeatedly folding short lengths of the structure andsecuring the folds. This type of structure is generally disclosed inSwieringa, U.S. Pat. No. 4,874,457, herein incorporated by reference.This corrugated absorbent structure can then be placed into an absorbentproduct having a liquid-permeable cover and a liquid-impermeable shellor barrier layer.

The invention also relates to a method of quickly drawing fluids deepinto an absorbent structure and locking them in the absorbent structure.The method includes the steps of (1) applying a liquid to an upperporous fibrous layer of a first planar region, (2) drawing the liquidfrom the upper layer into a lower layer of the first planar region, (3)allowing the liquid to transfer to an upper porous layer of a secondplanar region comprising hydrophilic fibers and a superabsorbentmaterial, and (4) drawing at least a portion of the liquid from theupper porous layer of the second planar region into a lower porous layerof the second planar region. Each layer of the absorbent structure hasan average pore size. The upper layer of the first planar region has afirst average pore size, the lower layer of the first planar region hasa second average pore size, the upper layer of the second planar regionhas a third average pore size, and the lower layer of the second planarregion has a fourth average pore size. The second average pore size isless than both the first and third average pore sizes, and the fourthaverage pore size is less than the third average pore size.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an exploded perspective view of an absorbent producthaving a pleated absorbent core of the present invention.

FIG. 2 depicts a partially cut-away perspective view of the absorbentstructure in sheet form.

FIG. 3 depicts a cross-sectional view of the absorbent structure of FIG.2.

FIG. 4 depicts a cross-sectional view of the absorbent core of FIG. 1.

FIG. 5 illustrates an exploded perspective view of an absorbent producthaving an alternative absorbent core of the present invention.

FIG. 6 depicts a partially cut-away perspective view of an absorbentstructure having three planar absorbent regions in sheet form.

FIG. 7 depicts a cross-sectional view of the absorbent structure of FIG.6.

FIG. 8 depicts a plan view of the strike-through plate of the testapparatus.

FIG. 9 is a side elevation of the strike-through plate of FIG. 8.

FIG. 10 is a second side elevation of the strike-through plate of FIG.8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a multilayered absorbent structurehaving a plurality of planar absorbent regions defined by decreasingpore size with increasing depth into the planar region. Generally, eachregion has an absorbent layer having relatively large average pore sizesat the top, liquid accepting surface, and a fluid management layerhaving relatively small average pore sizes at the lower surface. Theremay be additional layers therebetween, each layer having an average poresize no greater than the layer immediately preceding it toward theliquid accepting surface of the planar absorbent region. A subsequentplanar absorbent region has a top, liquid accepting surface with anaverage pore size which is larger than the fluid management layer at thelower surface of the previous absorbent region. Again, this region mayinclude additional layers as described above. Thus, each planar regionis defined by generally decreasing average pore size with increasingdepth. An increase in average pore size signals the beginning of anotherplanar absorbent region.

It is believed that the use of multiple layers in an absorbent structurehaving first relatively large then relatively small average pore sizesimproves the sequestering of liquids within the absorbent structure. Therelatively large pore sizes of the upper, fluid-accepting layers allowthese layers to accept gushes of liquid which are then drawn furtherinto the structure by gravity and capillary suction. The second planarregion acts to positively draw the liquids accepted by the first,body-facing planar absorbent region further into the structure. Thefluid management portion of the second planar absorbent region providesa capillary pressure gradient which assists the upper portion of thesecond planar absorbent region in drawing fluid from the relativelysmall pores of the lower layer of the first planar absorbent region.

Pore Size Measurement

There are a number of available techniques useful to measure the averagepore size of a nonwoven material. These techniques include the use ofthe liquid extrusion cell, developed at Textile Research Institute,Princeton, N.J., USA. This technique has been described in Miller etal., "An Extended Range Liquid Extrusion Method for Determining PoreSize Distributions", Textile Research Journal, Vol. 56, pp 35-40(1986),herein incorporated by reference, and it was used to derive amathematical model to predict the average pore size of a nonwovenfabric, Cohen, "A Wet Pore-Size Model for Coverstock Fabrics", Book ofPapers: The International Nonwoven Fabrics Conference, INDA-TEC'90,Association of the Nonwoven Fabrics Industry, pp. 317-330(1990), hereinincorporated by reference. Based on this model, the following equationwas used in the determination of average pore sizes reported in thespecification:

    r=(Σ.sub.i x.sub.i a.sup.2 /Σ.sub.i x.sub.i a)((ρ.sub.f /ξρ.sub.w)-1)/τ                                (I)

wherein

r is the average pore radius;

a is the fiber radius;

x is a number fraction;

ξ is the ratio of dry fabric density to wet fabric density;

ρ_(f) is the fiber density;

ρ_(w) is the dry fabric density; and

τ is the tortuosity parameter.

Based upon Cohen's work, the ratio 1.2 was selected for ξ, and 1.44 wasselected as τ.

The Absorbent Structure

The absorbent structure of the present invention can be used in anabsorbent device as illustrated in an exploded view in FIG. 1. In thisFigure, there is illustrated a cover sheet 10, an absorbent core 12, andliquid-impervious shell 14. The absorbent core 12 is corrugated toprovide a plurality of pleats 16, and the pleats 16 are secured with afacing material 18. The absorbent core 12 may be compressed along itslongitudinal edges to fit into the shell 14. The cover sheet 10 may thenbe secured around the perimeter of the shell 14 to provide the absorbentdevice.

The pleated absorbent core 12 is illustrated in greater detail in FIG.4. In this Figure, the facing material 18 can be seen which contacts andis affixed to the pleats 16 along their top ridges. The four layers ofthe inner structure of the absorbent core 12 can also be seen. The firstlayer 20 forms the top, liquid accepting layer of the absorbent core 12.The second layer 22 forms the lower layer of the first planar region inthis embodiment. The second planar region includes the third layer 24and the fourth layer 26. This preferred absorbent core 12 is illustratedin a sheet form in FIG. 2. Again, the first and second layers, 20 and 22form the first planar region 28, and the third and fourth layers 24 and26 form the second planar region 30. FIG. 3 is a sectional view of thesheet of FIG. 2. This again shows the first planar region 28 formed bythe first and second layers 20 and 22 and the second planar region 30formed by the third and fourth layers 24 and 26. In addition, thisFigure shows an optional fifth layer 32 which may be included as thebottom of the second planar region 30.

FIG. 5 illustrates an alternative embodiment of the absorbent product ofFIG. 1. In this embodiment, the cover sheet 50 and theliquid-impermeable shell 52 remain essentially unchanged. However, theabsorbent core 54 is formed from a flat sheet of absorbent material. Forexample, the absorbent sheet of FIGS. 2 and 3 may be used, or theabsorbent sheet of FIGS. 6 and 7 below, may be used.

FIGS. 6 and 7 illustrate yet another embodiment of the presentinvention. FIG. 6 illustrates a sheet-like absorbent structure 60 havingthree planar regions. The first planar region 62 is formed of an upperlayer 64 and a lower layer 66. The second planar region 68 is formed ofan upper layer 70 and a lower layer 72. The third planar region 74 isagain formed of an upper layer 76 and a lower layer 78. FIG. 7illustrates a sectional view of the structure of FIG. 6.

First Layer

At the body-facing, upper liquid accepting surface, there is a firstlayer comprising hydrophilic fibers. These fibers may be synthetic ornatural fibers or a combination of both. Preferably, the fibers of thefirst layer are hydrophilic synthetic fibers. These fibers may be formedfrom hydrophilic polymers, or they may be surfactant-treated hydrophobicfibers. The fibers of the first layer may also be a mixture ofhydrophilic and hydrophobic fibers combined in proportion to render thefirst layer generally hydrophilic.

A representative, nonlimiting list of synthetic fibers which may be usedin the present invention includes polyolefin (polyethylene,polypropylene), polyester, rayon, viscose, acrylic, or nylon fibers. Inaddition, the synthetic fibers can be copolymeric fibers or bicomponentfibers having, e.g., a polyester core and a polyethylene sheath.Bicomponent fibers may be formed from pairs of compatible polymerswherein the inner polymer has higher softening and melting points thanthe polymer forming the outer sheath. Preferably, the synthetic fibershave a denier of about 1.2 to 15.

A representative, nonlimiting list of natural fibers useful in thepresent invention includes hemp, jute, cotton, wood pulp, peat moss andthe like. These natural fibers may be modified natural fibers, e.g.Weyerhauser HBA and CCF wood pulp fibers, and the like.

The first layer comprises about 10 to 100 wt-% synthetic fibers and 0 to90 wt-% natural fibers. More preferably, the synthetic fibers comprisethe majority of the first layer, and most preferably, the first layer isabout 100 wt-% synthetic fibers. The first layer may have an averagepore size of about 10 to 1000 μm. In embodiments having both syntheticand natural fibers in any layer, the first layer preferably has anaverage pore size of about 75 to 400 μm, preferably about 100 to 300 μm,and most preferably about 190 to 250 μm. In embodiments having syntheticfibers in all layers, the first layer preferably has a pore size ofabout 75 to 1000 μm, preferably about 175 to 600 μm, and most preferablyabout 185 to 500 μm. The thickness of the first layer can range broadlydepending upon the intended use and construction of the absorbentstructure. The layer can range from about 0.01 to 0.25 inches inthickness. The layer will generally be thinner if the absorbentstructure is to be corrugated or pleated and thicker if the structure isto be used without folding.

Second Layer

Away from the body and in fluid communication with the first layer,there is a second, air-laid layer comprising hydrophilic fibers. Thesefibers may be synthetic or natural fibers or a combination of both.Preferably, the second layer comprises a mixture of hydrophilicsynthetic fibers and natural fibers. The synthetic fibers may be formedfrom hydrophilic polymers, or they may be surfactant-treated hydrophobicfibers. The fibers of the second layer may also be a mixture ofhydrophilic and hydrophobic fibers combined in proportion to render thesecond layer generally hydrophilic. The synthetic fibers listed abovefor the first layer may also be used in the second layer. Preferably,the synthetic fibers have a denier of about 1.2 to 15. The naturalfibers listed above for the first layer may also be used in the secondlayer.

The second layer comprises about 10 to 100 wt-% synthetic fibers and 0to 90 wt-% natural fibers. More preferably, the synthetic fiberscomprise the about 40 to 60 wt-% and the natural fibers comprise about60 to 40 wt-% of the second layer. The second layer may have an averagepore size of about 10 to 500 μm. In embodiments having both syntheticand natural fibers in any layer, the second layer preferably has anaverage pore size of about 40 to 140 μm, preferably about 60 to 120 μm,and most preferably about 80 to 100 μm. In embodiments having syntheticfibers in all layers, the second layer preferably has a pore size ofabout 10 to 500 μm, preferably about 50 to 180 μm, and most preferablyabout 100 to 150 μm. The thickness of the second layer can range broadlydepending upon the intended use and construction of the absorbentstructure. The layer can range from about 0.01 to 0.25 inches inthickness. The layer will generally be thinner, e.g., about 0.01 to 0.03inches, if the absorbent structure is to be corrugated or pleated andthicker, e.g., about 0.1 to 0.25 inches, if the structure is to be usedwithout folding.

Third Layer

Away from the body and in fluid communication with the second layer,there is a third layer comprising hydrophilic fibers. Again, thesefibers may be synthetic or natural fibers or a combination of both.Preferably, the third layer comprises air-laid hydrophilic syntheticfibers. The synthetic fibers may be formed from hydrophilic polymers, orthey may be surfactant-treated hydrophobic fibers. The fibers of thethird layer may also be a mixture of hydrophilic and hydrophobic fiberscombined in proportion to render the third layer generally hydrophilic.The synthetic fibers listed above for the first layer may also be usedin the third layer. Preferably, the synthetic fibers have a denier ofabout 1.2 to 15, more preferably, the fibers have a denier of about 3 to15. The natural fibers listed above for the second layer may also beused in the third layer.

The third layer preferably also comprises superabsorbent materials.Superabsorbent materials used in sanitary and incontinence products arewell known in the art. These materials include polyacrylates; modifiednatural and regenerated polymers such as polysaccharides; hydrocolloidssuch as modified polyacrylonitrile compounds; cross-linked nonionicpolymers such as polyoxyethylene, polyoxypropylene and mixture thereof;derivatives of isobutylene-maleic anhydride copolymers; copolymers suchas those disclosed in U.S. Pat. No. 4,880,868, available from ARCO; andnaturally occurring materials such as gums including guar gums, acaciagums, locust bean gums, and the like. The superabsorbent material may bepowdered or in fiber form. Preferably, the superabsorbent material is apowdered polyacrylate superabsorbent.

The third layer comprises about 10 to 100 wt-% synthetic fibers, 0 to 90wt-% natural fibers, and 0 to 80 wt-% superabsorbent material. Morepreferably, the synthetic fibers comprise about 20 to 60 wt-%, thenatural fibers comprise about 20 to 60 wt-%, and the superabsorbentmaterials comprise about 15 to 60 wt-% of the third layer. The thirdlayer may have an average pore size of about 75 to 1000 μm. Inembodiments having both synthetic and natural fibers in any layer, thethird layer preferably has an average pore size of about 400 to 1000 μm,preferably about 500 to 900 μm, and most preferably about 600 to 800 μm.In embodiments having synthetic fibers in all layers, the third layerpreferably has a pore size of about 75 to 1000 μm, more preferably about175 to 600 μm, and most preferably about 185 to 500 μm. The thickness ofthe third layer can range broadly depending upon the intended use andconstruction of the absorbent structure. The layer can range from about0.01 to 0.25 inches in thickness. The layer will generally be thinner,e.g., about 0.01 to 0.03 inches, if the absorbent structure is to becorrugated or pleated and thicker, e.g., about 0.1 to 0.25 inches, ifthe structure is to be used without folding.

Fourth Layer

Away from the body and in fluid communication with the third layer,there is a fourth layer comprising hydrophilic fibers. While this layermay be wet-laid, it is preferably an air-laid layer. The fibers used inthis layer may be synthetic or natural fibers or a combination of both.Preferably, the fourth layer comprises a mixture of hydrophilicsynthetic fibers and natural fibers. The synthetic fibers may be formedfrom hydrophilic polymers, or they may be surfactant-treated hydrophobicfibers. The fibers of the fourth layer may also be a mixture ofhydrophilic and hydrophobic fibers combined in proportion to render thefourth layer generally hydrophilic. The synthetic fibers listed abovefor the first layer may also be used in the fourth layer. Preferably,the synthetic fibers have a denier of about 1.2 to 15. The naturalfibers listed above for the third layer may also be used in the fourthlayer.

The fourth layer may also comprise superabsorbent materials. Usefulsuperabsorbent materials are listed above. Preferably, thesuperabsorbent material is a powdered polyacrylate superabsorbent.

The fourth layer comprises about 10 to 100 wt-% synthetic fibers, about0 to 90 wt-% natural fibers and about 0 to 80 wt-% of a superabsorbentmaterial. More preferably, the synthetic fibers comprise the about 20 to60 wt-%, the natural fibers comprise about 20 to 60 wt-% and thesuperabsorbent material comprises about 15 to 60 wt-% of the fourthlayer. The fourth layer may have an average pore size of about 10 to 500μm. In embodiments having both synthetic and natural fibers in anylayer, the fourth layer preferably has an average pore size of about 40to 140 μm, preferably about 60 to 120 μm, and most preferably about 80to 100 μm. In embodiments having synthetic fibers in all layers, thefourth layer preferably has a pore size of about 10 to 500 μm,preferably about 50 to 180 μ m, and most preferably about 100 to 150 μm.The thickness of the fourth layer can range broadly depending upon theintended use and construction of the absorbent structure. The layer canrange from about 0.01 to 0.25 inches in thickness. The layer willgenerally be thinner, e.g., about 0.01 to 0.03 inches, if the absorbentstructure is to be corrugated or pleated and thicker, e.g., about 0.1 to0.25 inches, if the structure is to be used without folding.

Subsequent Layers

While the present invention is described herein with reference to twoplanar regions comprising four layers in all, in certain applications,it may be helpful or necessary to include additional layers. Theselayers should generally be included in planar regions to result in theabsorbent structure having the following progression of relative poresizes: large, small, large, small, large, small, etc. Of course, theremay be one or more planar regions having the following progression ofpore sizes: large, medium, small; large, large, small; large, small,small; and the like. In addition, the subsequent planar absorbentregions may also incorporate superabsorbent materials.

How to Make

The multilayered absorbent structure of the present invention can beformed by laminating or combining individual nonwoven fabric webs, or itcan be prepared by forming several different nonwoven fabric layers in acontinuous process. Preferably, the absorbent structure is prepared bycombining two separate nonwoven structures having several layers toprovide the plurality of planar regions having a decreasing pore sizegradient within each planar region.

The absorbent webs which make up the planar regions can be prepared bymethods known to those of ordinary skill in the art, using conventionalgrinding equipment such as M&J equipment available from M&J Company ofSweden, Williams mills, Fitzmills, available from Fitzpatrick Co., and adual rotor webber, available from J. D. Hollingsworth on Wheels, Inc.,Greenville, S.C. In addition, synthetic fiber-rich structures can bethermobonded, melt-blown, spunbonded, formed with conventional air-laidequipment, such as card-and-bind equipment, and the like.

In a preferred embodiment, two fabric webs are combined to form anabsorbent structure of the present invention having two planar regionsof decreasing average pore size.

The first fabric web, which forms the bottom portion of the absorbentstructure, may include an optional bottom layer of 0.15 oz/yd², 1.8denier synthetic bicomponent fiber (BASF 1051, polyethylene overpolyester terephthalate (PET), available from BASF Fibers, Williamsburg,Va.), a second layer of a uniform blend of 0.4 to 0.6 oz/yd², 3 deniersynthetic bicomponent fiber (BASF 1050, polyethylene over PET) and 1 to1.5 oz/yd² cellulosic pulp fibers (Rayonier Pulp E-Type, available fromRayonier, Inc., Stamford, Conn.), and an upper layer of a uniform blendof 0.1 oz/yd², 10 denier synthetic bicomponent fiber (BASF 1088,polyethylene over PET) and 0.1 oz/yd², 15 denier synthetic PET fiber(duPont 374W, available from E. I. duPont de Nemours, Textile FibersDepartment, Wilmington, Del.). The upper layer is carded onto a meshbelt. The synthetic fiber/pulp layer is then formed in a dual rotorwebber on the upper layer, and the optional synthetic fiber layer may becarded on the synthetic fiber/pulp layer. The structure is thenthermally bonded in an oven.

The second fabric web, which forms the upper portion of the absorbentstructure, includes a bottom layer of a uniform blend of 0.1 oz/yd², 10denier synthetic bicomponent fiber (BASF 1088) and 0.1 oz/yd², 15 deniersynthetic PET fiber (duPont 374W), a second layer of a uniform blend of0.4 oz/yd², 3 denier synthetic bicomponent fiber (BASF 1050) and 0.5oz/yd² cellulosic pulp fibers (Rayonier Pulp E-Type), and an upper layerof about 0.4 oz/yd², 3 denier synthetic bicomponent fiber (BASF 1050).The upper layer is again carded onto a mesh belt. The syntheticfiber/pulp layer is then formed is a dual rotor webber on the upperlayer, and the 3 denier bicomponent fiber layer is carded on thesynthetic fiber/pulp layer. This structure is also thermally bonded inan oven.

The two webs are then combined in an absorbent structure manufacturingprocess. The bottom web is first unwound onto a carrier belt. Asuperabsorbent powder and optional materials including odor controlpowder or liquid can be deposited onto the top surface of the bottomweb. Next, the top web is unwound onto the bottom web. This web may beadhered to the bottom web. The edges, and optionally a random patternacross the web, may then be embossed to increase the integrity of theabsorbent structure. The embossing may be performed with or withoutadded heat.

The contact of the two fabric webs forms an absorbent structure havingtwo superposed planar regions. The upper planar region includes, fromthe top surface, (1) a 3 denier synthetic bicomponent fiber layer and(2) a blended layer of 3 denier synthetic bicomponent fiber andcellulosic pulp. The lower planar region includes, from the uppersurface, (1) the merged synthetic bicomponent fiber/synthetic PET fiberweb of the first and second fabric webs comprising superabsorbentpowder, (2) a blended layer of 3 denier synthetic bicomponent fiber andcellulosic pulp and (3) the optional 1.8 denier synthetic bicomponentfiber layer. The formation of the absorbent structure is completed withthe embossing of the structure.

The absorbent structure may then be further processed into an absorbentarticle. This processing includes corrugating the structure, applying abicomponent bonding veneer, and heating the corrugated structure withthe bonding veneer to thermobond the veneer to the structure to maintainthe corrugation. The bonding veneer may be, for example, 0.5 oz/yd², 3denier synthetic bicomponent fiber (BASF 1050). After the structure hasbeen bonded, the edges of the structure may be embossed to seal them.This corrugation is taught in Swieringa, U.S. Pat. No. 4,874,457. Amasking layer may be adhered to the upper surface of the bonding veneer,and absorbent pads may be slit from the structure. The absorbent padsare then placed into a formed foam shell and covered, for example, withnonwoven web of 0.7 oz/yd², 3 denier synthetic bicomponent fiber (BASF1050) facing veneer, and the products are cut from the foam shellmaterial web. While the above method of making relates to a particularprocess and product, it will readily be recognized that this process canbe modified, e.g. by adding additional powders, etc, to thesuperabsorbent-containing layer, by adding additional planar absorbentregions, and the like. In addition, the products may be manufactured ina single line, or in multiple manufacturing lines. The products may beformed by stamping out the products via conventional cutting methods,e.g., die cutting, flying knives, and the like.

How to Use

Using the method described above or other processes known to those ofordinary skill in the art, the absorbent structure may be incorporatedinto any suitable absorbent product such as a sanitary napkin, diaper,adult incontinence device, and the like. In one preferred embodiment,the absorbent structure is incorporated into an adult incontinencedevice described in the commonly assigned application, Poccia et al.,U.S. Ser. No. 08/184,402, filed Jan. 20, 1994, now pending hereinincorporated by reference. In such a device, the absorbent structure ispreferably corrugated and bonded to a layer of nonwoven fabric tomaintain the stability of the structure. The corrugated structure may becombined with additional fluid management layers and masking layersbefore being enclosed by a cover layer and a fluid impervious backingshell.

Of course, the absorbent structure need not be corrugated or pleated inuse in an absorbent product. For example, FIGS. 2, 3, 6 and 7 illustratea thicker absorbent structure useful in absorbent products.

Test Method Used

The following test method can be used to measure properties of absorbentstructures according to the present invention:

PENETRATION TIME AND INCREMENTAL SURFACE WETNESS

The test method measures the penetration time and surface wetness. Thepenetration time is the time taken for a given amount of liquid to passthrough the facing and be absorbed into an absorbent structure. Surfacewetness measures the amount of liquid that passes from the standard corematerial through a cover facing, to cause wetness on the surface of theproduct.

This test uses the following equipment:

1. Calibrated stopwatch/timer accurate to 0.1 seconds.

2. Calibrated top loading balance accurate to 0.01 grams.

3. Ringstand and clamps.

4. Separatory funnel with Nalgene stopcock which is calibrated to adischarge flow rate of 20±1 ml/second.

5. Filter paper--Eaton-Dikeman #90-140, available from Eaton-Dikeman,Division of Know/Ton Brothers, Mt. Holly Springs, Pa. 17065-0238.

6. Test apparatus consisting of the following:

(a) Electronic timer measuring to 0.01 seconds. Available through J. G.McGuffey & Company, Peachtree City, Ga.

(b) Strike-through plate:

A 25 mm thick acrylic square strike-through plate is shown in FIGS.8-10. The dimensions given in this figure are in mm. The strike-throughplate 80 has marginal areas 82 which are available for optionalweighting strips 84. Wire electrodes 86 having a diameter of 1 mm areplaced in square-cut grooves 88 and may be secured with quick settingepoxy resin. These electrodes 86 must be kept clean. The total weight ofthe plate must be 225 ±5grams. Care must be taken that the electrodes

86 are positioned as specified. The strike-through plate is availablethrough R & L Engineering, Albany, Ga.

7. Surface wetness apparatus consisting of a compression weight whichhas a total weight equivalent to 0.5 psi (4.4 lbs), with a 4 inch by 2inch base.

8. Tensiometer, DuNouy, available through Central Scientific Company,Chicago, Ill. or equivalent.

9. A 6 inch steel ruler.

10. A liquid container with a minimum capacity of 200 ml.

The test also uses a 1.59% Saline Test Solution prepared from deionizedwater.

Test Procedure

Begin testing by calibrating the flow rate of the separatory funnel.Using the Fisher Model 21 Tensiomat, record the surface tension of thetest solution. Weigh the product to be tested and record the weight to0.01 g. This is the "pre-weigh". Weigh overflow paper, e.g., paper towelor filter paper, and record the weight. Place the product on theoverflow paper. If overflow ultimately occurs, reweigh the overflowpaper to determine the amount of test fluid not absorbed into the testproduct. Place the strike-through plate over the center of the product.Attach a 2" length of tubing to the tip of the funnel. Tubing should be1" above the product. Center the separatory funnel over the plate.Assure that the apparatus is in the middle and center of the product andthat the plate contacts the surface of the product being tested. Theextending electrodes should run parallel with the products length.

Tare a liquid container on the top loading balance. Measure desiredamount test fluid into the container. Apply 40 ml of test fluid to theproducts at a flow rate of 20 cc/second. Repeat test with unusedproducts with 60 ml of test fluid and 80 ml of test fluid with a flowrate of 20 cc/second. Ensure that the stopcock on the calibratedseparatory funnel is closed. Ensure the electrodes on the strike-throughplate are connected to the timer. Switch on the timer and reset to zero.Add the test fluid to the calibrated separatory funnel. Open thestopcock and discharge the test fluid. The initial flow of liquid willcomplete the electrical circuit and start the timer. The timer will stopwhen the liquid is absorbed into the test product and fallen below thelevel of the electrodes. As soon as the separatory funnel has beenemptied, start the stopwatch/timer and time for 15 sec. Record thepenetration time from the electronic timer in seconds to 0.01 sec.Remove test apparatus and weigh the tested product. Record the weight.This is recorded as the product post-weight.

When 15 seconds have elapsed, center a set of two 4"×2" pre-weighedfilter papers on the product. The two filter papers are stacked one onthe other. Apply the compression weight on top of the filter paper andrestart start stopwatch/timer. Allow sample to remain under thecompression weight for two minutes. Remove weight and filter paper.Weigh the filter paper to the nearest 0.01 gram. Record weight. Repeatthe steps of this paragraph until less than 1 g of liquid is recorded.

Calculations

Calculate the fluid absorbed by subtracting the product pre-weight fromthe product post-weight. Record the value for fluid absorbed. Calculatethe fluid absorbency rate by dividing the fluid absorbed by the time.Record this value in grams/sec to 0.1 g/sec. Calculate the amount ofoverflow by subtracting the overflow dry weight from the overflow wetweight. This is recorded as overflow amount. Calculate the difference inweight between each set of wet and dry filter papers to the nearest 0.01gram. Continue to calculate the differences for each set of filterpapers used using the following formula:

    Wet Filter Paper(g)-Dry Filter Paper(g)=Wetness Extracted(g)(II)

Record the sum of the differences from Calculation (II) as the totalamount of surface wetness. The results of these measurements andcalculations result in the strike-through and surface wetness datarecorded in the Examples.

EXAMPLES

The present invention will be further understood by reference to thefollowing specific Examples which are illustrative of the composition,form and method of producing the multilayered absorbent structure of thepresent invention. It is to be understood that many variations ofcomposition, form and method of producing the absorbent structure wouldbe apparent to those skilled in the art. The following Examples, whereinparts and percentages are by weight unless otherwise indicated, are onlyillustrative.

EXAMPLE 1

Four nonwoven fabric layers were chosen to illustrate the presentinvention. Five samples of an absorbent structure of the presentinvention were set up with the following pore size sequence, startingwith the topsheet:

    ______________________________________                                        Experimental                                                                  Layer No.   Description                                                                              Average Pore Size                                      ______________________________________                                        1           Bicomponent                                                                              200 μm                                                          Fabric A                                                          2           Fabric B   140 μm                                              3           Bicomponent                                                                              190 μm                                                          Fabric C                                                          4           Fabric B   140 μm                                              ______________________________________                                    

The characteristics of the fabric layers are as follows:

Bicomponent Fabric A: a soft, thermobonded, semiabsorbent materialmanufactured from 3 denier bicomponent fibers (BASF 1050) having athickness of about 30 mils (0.030 in) and a basis weight of about 0.7oz/yd².

Fabric B: a soft, card-and-bind, blend of 6 denier and 3 denierpolyester, semiabsorbent material having a thickness of about 0.017inches and a basis weight of about 0.65 oz/yd² (Fiber Tech 68798,available from Fiber Tech, Rogers, Ark.).

Bicomponent Fabric C: a soft, thermobonded, semiabsorbent materialmanufactured from 3 denier bicomponent fibers (BASF 1050) having athickness of about 20 mils (0.020 in) and a basis weight of about 0.5oz/yd².

Five samples of a control configuration were also assembled with theconventional single funnel approach:

    ______________________________________                                        Control                                                                       Layer No.   Description                                                                              Average Pore Size                                      ______________________________________                                        1           Bicomponent                                                                              200 μm                                                          Fabric A                                                          2           Bicomponent                                                                              190 μm                                                          Fabric C                                                          3           Fabric B   140 μm                                              4           Fabric B   140 μm                                              ______________________________________                                    

Each of these samples was placed over filter board, which stimulates anabsorbent core, and then was tested for strike-through (penetration)time and for rewet (surface wetness). The test method is describedabove. The dosage in these tests was 15 ml of saline solution. The rewettest was performed by placing pre-weighed 2"-4" cuts of Eaton-Dikeman#901-140 filter paper on top of the absorbent sample and then placing a2"×4"weight on top of the filter paper. The weight represents an appliedpressure of 0.5 psi. After 2 minutes, the filter paper was reweighed andthe difference was reported as the rewet.

The results were as follows:

    ______________________________________                                                        Experimental                                                                           Control                                              ______________________________________                                        Strike-through time (sec.):                                                                     2.98       3.32                                             Std. Dev. (sec.): 0.25       0.28                                             Rewet (g):        0.58       1.22                                             Std. Dev. (g):    0.21       0.32                                             ______________________________________                                    

The results show that with the absorbent structure of the presentinvention, the rewet is substantially reduced compared with the control.The difference is statistically significant with 95% confidence. Also,the strike-through time is reduced with the absorbent structure of thepresent invention versus the control. This difference too isstatistically significant.

Thus, a novel arrangement of layer has been achieved such that both thestrike-through time and the rewet are reduced, improving productperformance.

EXAMPLE 2

A control absorbent structure having a negative pore size gradient in apleated absorbent core as described in Swieringa, U.S. Pat. No.4,874,457, was prepared having the layers with the porosity identifiedbelow:

    ______________________________________                                        Control                                                                       Layer No.   Description                                                                              Average Pore Size                                      ______________________________________                                        1           Bicomponent                                                                              190 μm                                                          Fabric A                                                          2           Bicomponent                                                                              190 μm                                                          Fabric A                                                          3           Fabric B   140 μm                                              4           Fabric D    60 μm                                              ______________________________________                                    

The characteristics of the fabric layers are as follows:

Bicomponent Fabric A: As in Example 1.

Fabric B: As in Example 1.

Fabric D: A homogeneous blend of Syn-Pulp (available from Temple Inland,Silsbe, Tex.) and Aquakeep J-550 (available from Sumitomo, Japan). Theblend is cast onto tissue and thermobonded.

This product also contains a powdered polyacrylate superabsorbent thethird layer adjacent the lower surface of the absorbent core, with thesuperabsorbent predominantly positioned near the base of the core.

Next, a pleated core structure was made, similar to the control, butusing the absorbent structure of the present invention (Product A). Thelayers of the core were as follows:

    ______________________________________                                        Product A                                                                     Layer No.  Description  Average Pore Size                                     ______________________________________                                        1          Fabric E, top                                                                              190 μm                                             2          Fabric E, middle                                                                           100 μm                                             3          Fabric E, bottom                                                                           770 μm                                                        and Fabric F, top                                                  4          Fabric F, bottom                                                                            60 μm                                             ______________________________________                                    

The characteristics of the fabric layers are as follows:

Fabric E: Fabric E has a bottom layer of a uniform blend of 0.1 oz/yd²,10 denier synthetic bicomponent fiber (BASF 1088) and 0.1 oz/yd², 15denier synthetic PET fiber (duPont 374W), a second layer of a uniformblend of 0.4 oz/yd², 3 denier synthetic bicomponent fiber (BASF 1050)and 0.5 oz/yd² cellulosic pulp fibers (Rayonier Pulp E-Type), and anupper layer of about 0.4 oz/yd², 3 denier synthetic bicomponent fiber(BASF 1050). This is described above in the specification as "the secondfabric web".

Fabric F: A bottom layer of a uniform blend of 0.4 to 0.6 oz/yd², 3denier synthetic bicomponent fiber (BASF 1050) and 1 to 1.5 oz/yd²cellulosic pulp fibers Rayonier Pulp E-Type, and an upper layer of auniform blend of 0.1 oz/yd², 10 denier synthetic bicomponent fiber (BASF1088) and 0.1 oz/yd², 15 denier synthetic PET fiber (duPont 374W). Thisis described above in the specification as "the first fabric web"without the optional 0.15 oz/yd² layer.

The same type of superabsorbent that was used in the control product wasincorporated into the third layer of the absorbent structure of ProductA.

The control and Product A were tested for Strike-through and re-wet asdescribed for Example 1. There were 5 products tested of each designusing a dosage of 40 ml. The results were as follows:

    ______________________________________                                                         Control                                                                              Product A                                             ______________________________________                                        Strike-through time (sec.):                                                                      3.58     2.44                                              Std. Dev. (sec.):  1.16     0.24                                              Rewet (g):         4.02     1.36                                              Std. Dev. (sec.):  1.95     0.46                                              ______________________________________                                    

The results show that with the absorbent structure of the presentinvention, the strike-through time and the rewet is substantiallyreduced compared to the Control. The difference is statisticallysignificant with 95% confidence.

The specification and examples above are presented to aid in thecomplete and non-limiting understanding of the invention disclosedherein. Since many variations and embodiments of the invention can bemade without departing from its spirit and scope, the invention residesin the claims hereinafter appended.

What is claimed is:
 1. An absorbent structure providing a substantiallydry liquid-accepting surface after application of a quantity of liquidto the surface, the structure comprising:a) a first planar regiondefining an upper, liquid-accepting surface of the absorbent structurecomprising:i) a fibrous upper layer having a first average pore size;and ii) an air-laid fibrous lower layer disposed below and in fluidcommunication with the upper layer, having a second average pore size,less than the first average pore size; and b) a second planar region,disposed below and in fluid communication with the lower layer of thefirst planar region, comprising:i) a fibrous upper layer having a thirdaverage pore size; and ii) a fibrous lower layer disposed below and influid communication with the upper layer of the second planar region,having a fourth average pore size, less than the third average poresize;wherein the third average pore size is greater than the secondaverage pore size.
 2. The absorbent structure of claim 1 wherein thefibrous upper layer of the first planar region comprises synthetichydrophilic fibers having a fineness of about 1.2 to 15 denier.
 3. Theabsorbent structure of claim 1 wherein the air-laid fibrous lower layerof the first planar region comprises a mixture of synthetic fibers andnatural fibers.
 4. The absorbent structure of claim 3 wherein theair-laid fibrous layer comprises about 10 to 100 wt-% of the syntheticfibers and about 90 to 0 wt-% of the natural fibers.
 5. The absorbentstructure of claim 3 wherein the synthetic fibers are hydrophilic fibershaving a fineness of about 1.2 to 15 denier.
 6. The absorbent structureof claim 3 wherein the natural fibers are wood pulp fluff.
 7. Theabsorbent structure of claim 1 wherein the upper fibrous layer of thesecond planar region comprises synthetic hydrophilic fibers having afineness of about 1.2 to 25 denier.
 8. The absorbent structure of claim1 wherein the upper fibrous layer of the second planar region comprisesabout 10 to 100 wt-% synthetic hydrophilic fibers, about 0 to 90 wt-%natural fibers and about 5 to 80 wt-% superabsorbent material.
 9. Theabsorbent structure of claim 8 wherein the superabsorbent material isselected from the group consisting of polyacrylates, polysaccharides,hydrocolloids, and naturally-occurring gums.
 10. The absorbent structureof claim 1 wherein the air-laid fibrous lower layer of the second planarregion comprises a mixture of about 10 to 100 wt-% synthetic fibers andabout 90 to 0 wt-% natural fibers.
 11. The absorbent structure of claim10 wherein the synthetic fibers are hydrophilic fibers having a finenessof about 1.2 to 15 denier.
 12. The absorbent structure of claim 10wherein the natural fibers are wood pulp fluff.
 13. The absorbentstructure of claim 1 which further comprises at least one absorbentlayer below and in fluid communication with the lower layer of thesecond planar region.
 14. The absorbent structure of claim 13 whichfurther comprises a third planar region below and in fluid communicationwith the lower layer of the second planar region, the third planarregion comprising:i) a fibrous upper layer having a fifth average poresize; and ii) an air-laid fibrous lower layer disposed below and influid communication with the upper layer of the third planar region,having a sixth average pore size, less than the fifth average poresize;wherein the fifth average pore size is greater than the fourthaverage pore size.
 15. The absorbent structure of claim 1 wherein thefirst planar region further comprises an absorbent layer disposedbetween and in fluid communication with the upper and lower layers ofthe first planar region and having an average pore size which is lessthan the first average pore size and which is at least as large as thesecond average pore size.
 16. The absorbent structure of claim 1 whereinthe second planar region further comprises an absorbent layer disposedbetween and in fluid communication with the upper and lower layers ofthe second planar region and having an average pore size which is lessthan the third average pore size and which is at least as large as thefourth average pore size.
 17. A corrugated absorbent structure formed bycorrugating the absorbent structure of claim
 1. 18. A method formanufacturing an absorbent structure which provides a substantially dryliquid-accepting surface after application of a quantity of liquid tothe surface, comprising the steps of:a) forming a first planar regiondefining an upper, liquid-accepting surface of the absorbent structurecomprising:i) a fibrous upper layer having a first average pore size;and ii) an air-laid fibrous lower layer disposed below and in fluidcommunication with the upper layer, having a second average pore size,less than the first average pore size; b) forming a second planarregion, comprising:i) a fibrous upper layer having a third average poresize; and ii) an air-laid fibrous lower layer disposed below and influid communication with the upper layer of the second planar region,having a fourth average pore size, less than the third average poresize; and c) superposing the first planar region over the second planarregion wherein the fibrous upper layer of the second planar region is influid communication with the air-laid lower layer of the first.
 19. Themethod of claim 18 which further comprises the step of applying asuperabsorbent powder material to the fibrous upper layer of the secondplanar region.